Conventionally, liquid chromatographs include a reservoir to store a mobile phase (elution solvent), a pump to supply the mobile phase from the reservoir, a sample injector to inject a sample together with the mobile phase into a tubing leading to a column, the column filled with a packing material for separating components in the sample, an oven to keep the column at a constant temperature, and a detector to detect the separated components in the sample. Of these, the sample injector is so structured as to attach a sample injection needle that has drawn in the sample by suction to a sample injection port (sample injection part) and to inject the sample together with the mobile phase into the tubing via a switching valve.
In recent years, with improvement in the detection sensitivity of liquid chromatographs, a phenomenon called carryover has become a problem. The carryover, which is a phenomenon that an earlier measured sample remains in a liquid chromatograph to present such a detection result as if the substance were present in a currently measured sample, degrades the reliability of analysis results. The carryover is caused by the mixture of a residual sample at the time of injecting the next sample, the residual sample having adhered to a metal and/or a resin inside a sample injector at the time of injecting the sample together with a mobile phase into a tubing and remained.
Therefore, in order to ensure reduction of the carryover, a technique has been proposed that provides two injection needles and attaches a first one of the sample injection needles that has drawn in a sample by suction to a sample injection port, thereby allowing the sample to be supplied to a column without intervention of a switching valve, thus preventing the sample from remaining in the switching valve as it does conventionally and making it possible to sufficiently reduce the carryover. (For example, see Patent Document 1.)
In the apparatus provided with two injection needles illustrated in Patent Document 1, a sample is injected through the process of disconnecting a first sample injection needle in a mobile phase supplying state where the first sample injection needle is connected to an injection part, and connecting a second sample injection needle retaining the sample to the injection and causing a mobile phase to restart flowing into a column.